FIG. 4 is a perspective view of a conventional disk recording or playback device. A chassis 1 is provided thereon with a turntable 3 for placing a disk 7 thereon as is well known, and a pickup 2 movable toward or away from the turntable 3. The pickup 2 has an objective 22 on its upper surface and is movable as guided by two guide rods 4, 4 on the chassis 1. Each guide rod 4 has opposite ends fitting in respective brackets 9, 9 on the chassis 1 in the vicinity of the turntable 3.
FIG. 5 is a rear view of the disk 7. Pits 73, 73 are formed in the signal bearing surface of the disk 7 circumferentially thereof. The intensity of light reflected from the disk differs when a laser beam is projected on the pit 73 and when the laser beam is projected on a portion other than the pit 73. This reproduces a digital signal comprising 0 and 1.
Available in recent years are disks 7 adapted to record signals thereon at a high density. These disks are smaller in the spacing D between pits 73, 73 in the radial direction of the disk shown in FIG. 5.
To read the signal recorded at the high density, the objective 22 is given a great numerical aperture (NA, e.g., 0.6) to reduce the diameter of the beam.
When a great numerical aperture is given, it becomes impossible to decrease the beam diameter if the optical axis of the laser beam tilts relative to the disk owing to the refraction of light, presenting difficulty in reading data.
It is further known that when the optical axis of the laser beam tilts slightly relative to the signal bearing surface of the disk 7, coma occurs in proportion to the third power of the numerical aperture and to the thickness of the disk. Coma distorts the waveform of reproduced signals to result in greater jitter. In other words, if the optical axis of the laser beam tilts relative to the signal bearing surface of the disk 7, serious jitter is liable to occur because of the great numerical aperture, hence a need to correct the tilt of optical axis of the laser beam.
It is therefore proposed to provide a mechanism on the chassis 1 for adjusting the tilt of the pickup 2 and to finely adjust the tilt of the pickup 2 during the process for fabricating the disk recording or playback device for smooth reproduction of signals which are recorded on disks at a high density.
FIG. 6 is a view in section taken along a plane containing the line A—A in FIG. 4 and showing the conventional tilt adjusting mechanism, with the pickup 2 omitted.
The two brackets 9, 9 are mounted on the chassis 1 and have respective openings 99 facing inward for the guide rod 4 to fit in. A screw bore 10 is formed in one of the brackets 9 on the chassis 1, and an adjusting screw 6 is driven into the screw bore 10 from below the chassis 1. The guide rod 4 is supported at one end thereof by a projection 97 on the other bracket 9 and at the other end thereof by the adjusting screw 6. Tension springs 40, 40 are attached to and extend between the guide rod 4 and the chassis 1 for biasing the rod 4 downward. The tension springs 40, adjusting screw 6 and brackets 9 provide the adjusting mechanism 5 for tilting the guide rod 4 and the pickup 2 in a plane perpendicular to the chassis 1 and containing the direction of movement of the pickup 2.
To tilt the pickup 2, the adjusting screw 6 is advanced by turning. The guide rod 4 pivotally moves upward from the solid-line position shown in FIG. 6 about the point S of contact between the rod 4 and the bracket 9 against the tension springs 40 as indicated by a chain line. The guide rod 4 may of course be pivotally moved downward. In this way, the tilt of the pickup 2 can be adjusted by an angle α.
However, the above mechanism still remains to be improved as will be described below.
Since the guide rod 4 pivotally moves about the point S of contact of the rod 4 with the bracket 9, the greater the distance of a point on the guide rod 4 from the contact point S, the greater the variation in the level of the point. The pickup 2, which moves along the guide rod 4, is the greatest distance away from the contact point S when the objective 22 on the pickup 2 is opposed to the outermost periphery of the disk.
With reference to FIG. 6, suppose the objective 22 is positioned at a point P on the guide rod 4 when the lens 22 reaches the outermost peripehry of the disk, and the distance from the point P to the contact point S is d. The guide rod 4 is tilted about the contact point S by tilt adjustment, so that at the point P where the objective 22 is the greatest distance away from the contact point S, the adjustment alters the level of the guide rod 4 by a value h given below.h=d×sin α
Because the brackets 9 are positioned in the vicinity of the turntable 3 as shown in FIG. 4, d, when greatest, is about 60 mm which is the radius of the disk. Since the angle α of adjustment of the guide rod 4 as measured by the present applicant is about 0.3 deg, h is:
 h=60×sin 0.3=0.26 mm
In order to focus the beam from the objective 22 on the disk to properly effect focusing servo, the objective 22 is made movable upward or downward by a very small amount. However, if great, the value h will be in excess of the this movable amount, entailing the likelihood that a correct focusing servo operation can not be performed.
An object of the present invention is to diminish the variations in the level of the guide rod which are involved in adjusting the tilt of the guide rod to ensure a correct focusing servo operation.